1. Field of the Invention
The present invention relates to vibration damping rubber compositions, more specifically, relates to vibration damping rubber compositions used on engine mounts or the like, which function as a support for automobile engines and suppress vibration transmission.
2. Description of the Art
In conventional vibration damping rubber compositions, for example, rubber compositions prepared by adding a sulfur-vulcanizing agent and a vulcanization accelerator to a rubber material, such as a natural rubber and a synthetic rubber, have been used. However, such rubber compositions have a disadvantage that the rubbers harden due to long-term service (heat aging) causing a decrease in the vibration damping performance.
The causes of such heat aging deterioration are thought as follows: (a) an increase in cross-linking density by a change in a form of sulfur cross-linking from a polysulfide bond to a disulfide or monosulfide bond; (b) oxidative deterioration of the rubber material itself; and (c) cross-linking caused by the sulfur-vulcanizing agent.
These problems have been dealt with by, for example, (a) forming a monosulfide-rich cross-link, (b) adding an antioxidant, and (c) minimizing the blend ratio of the sulfur-vulcanizing agent. However, the hardening of the rubber due to long-term service (heat aging) could not be sufficiently suppressed. Therefore, the vibration damping performance was poor.
Consequently, in order to solve the problems in the vibration damping performance, a variety of rubber compositions using a bismaleimide-based compound as the vulcanizing agent have been proposed (for example,
Japanese Unexamined Patent Publication No. 2-284935,
Japanese Unexamined Patent Publication No. 4-136049, and
Japanese Patent Publication No. 7-122006).
However, each of the rubber compositions described in the above-mentioned three Patent Publications includes a sulfur-vulcanizing agent along with a bismaleimide-based compound. Therefore, the cross-linking progresses by the sulfur-vulcanizing agent to harden the rubber. Thus, such rubber compositions have a disadvantage that sufficient vibration damping performance cannot be achieved.
The present invention has been made in view of the above-mentioned circumstances, and it is an object of the present invention to provide a vibration damping rubber composition which can achieve excellent vibration damping performance.